Police have closed part of the River Ouse in York to all traffic this evening and have cordoned off two stretches of the bank.
Officers are diverting walkers and cyclists off the footpath on the north bank between Scarborough Bridge and Clifton Bridge, and boats are being turned round at Scarborough Bridge.
They were not saying why they were carrying out the operation which is believed to involve at least a dozen officers and police staff.
A search and rescue boat was seen on the water earlier this afternoon near the south side near undergrowth opposite St Peter's School's playing fields.
Police had cordoned off a stretch of the river bank which has several small bays by some willow trees on the Acomb side and were taking photographs.
The police activity began shortly after 4pm when couples, families and strollers were enjoying the warm weather.
A group of young people nearby said there had been no warning before the police arrived and no indication that anything was out of the ordinary.
Boats north of Clifton Bridge are unable to go downstream and at least one motorboat wanting to go upstream had to perform a U-turn at Scarborough Bridge. Police officers were on the towpath at the river's edge to make sure boats did not go further.
Police routinely close the river if they are expecting to use divers or carry out other operations on the water.
More to follow.